As disclosed in Patent Documents 1 and 2 below, a transmission device including an eccentric shaft, an eccentric unit and a rotary unit as follows is already known. The eccentric shaft includes a main shaft portion on a first rotary axis and an eccentric shaft portion on a second rotary axis disposed eccentrically with respect to the first rotary axis. The eccentric shaft portion is capable of revolving about the first rotary axis. The eccentric unit is rotatably supported on the eccentric shaft portion of the eccentric shaft and is capable of revolving about the first rotary axis, while rotating around the second rotary axis in a manner of being operatively associated with the rotation of the eccentric shaft about the first rotary axis. The rotary unit is capable of rotating about the first rotary axis, while being operatively associated with the axial rotation and the orbital revolution of the eccentric unit. In the transmission device, rotation of the main shaft portion of the eccentric shaft is transmitted to the rotary unit with speed reduced via a curved wave groove and a plurality of intermediate members having an orbit restricted by the curved wave groove.
Specifically, the arrangement disclosed in Patent Document 1 includes an eccentric rotary unit (eccentric unit) 4 rotatably supported on an eccentric shaft 22 of an eccentric shaft that includes an input shaft (main shaft portion) 2 extending along a first rotary axis and the eccentric shaft (eccentric shaft portion) 22 extending along a second rotary axis. The arrangement further includes a plurality of rolling balls (rolling elements) 53 that move along an orbit defined by an inner groove (curved wave groove) 51 in an outer periphery of the eccentric rotary unit 4 and an outer groove (curved wave groove) 52 in an inner periphery of a housing 1. The movement of the rolling balls 53 along the orbit causes the eccentric rotary unit 4 to revolve about the first rotary axis, while rotating around the second rotary axis, in a manner of being operatively associated with rotation of the input shaft 2. The arrangement further includes a driven disk (rotary unit) 32 that includes an adjustment mechanism 40 disposed on a side surface opposing one side surface of the eccentric rotary unit 4. The driven disc 32 rotates about the first rotary axis on the basis of the axial rotation and the orbital revolution of the eccentric rotary unit 4. (It is noted that the reference numerals and symbols used in the description of Patent Document 1 refer to those used in Patent Document 1.) Further, Patent Document 2 discloses an arrangement that includes an eccentric plate (eccentric unit) 4 rotatably supported on an eccentric shaft 12 that includes an input shaft portion (main shaft portion) 12a extending along a first rotary axis and an eccentric portion (eccentric shaft portion) 12d extending along a second rotary axis. The arrangement further includes a plurality of first rolling balls (rolling elements) 10 that move along an orbit defined by an epicycloidal groove portion (curved wave groove) 7 in one side surface of the eccentric plate 4 and a hypocycloidal groove (curved wave groove) 6 in a side surface of a stationary plate 3 that opposes the eccentric plate 4. The movement of the first rolling balls 10 along the orbit causes the eccentric plate 4 to revolve about the first rotary axis, while rotating around the second rotary axis, in a manner of being operatively associated with rotation of the input shaft portion 12a. The arrangement further includes a plurality of second rolling balls (rolling elements) 11 that move along an orbit defined by a hypocycloidal groove portion (curved wave groove) 8 in the other side surface of the eccentric plate 4 and a epicycloidal groove (curved wave groove) 9 in a side surface of an output plate (rotary unit) 5 that opposes the eccentric plate 4, on the basis of the axial rotation and the orbital revolution of the eccentric plate 4. The movement of the second rolling balls 11 along the orbit causes the output plate 5 to rotate about the first rotary axis in a manner of being operatively associated with the axial rotation and the orbital revolution of the eccentric plate 4. (It is noted that the reference symbols used in the description of Patent Document 2 refer to the reference symbols used in Patent Document 2.)